The mother, killed along with her infant sons a few days ago, lived in a town that knows what it is doing when it comes to preventing domestic violence, and takes pains to track those in harm’s way. Still, here we are again: Trying to make sense of yet another incomprehensible act by a man who wouldn’t let go.

Arlington, together with Cambridge and Belmont, has a special high-risk team whose members identify people most likely to be harmed by their partners.

Those in what team director Cindy Nemet calls “imminent danger of harm or lethality” are placed on a list, and police and social workers in all three communities share resources and provide extra services to keep them safe.

There are a mind-boggling 134 people on that three-community list right now. That’s 134 people who could be killed by their partners. And 35 of them live in Arlington.

Mei Jones wasn’t one of them. It’s not clear if she was on the radar of anyone who might have seen how much danger she was in.

Her brother had no inkling his sister might have been abused. “We never heard anything like that,” he told the Globe’s Patricia Wen. Neighbors knew something was not right about Scott Jones, who was house-proud in a way that veered into the obsessive. He had been seen loading his belongings into a car the Saturday before the whole family was found dead on Monday. But none of the neighbors who spoke to reporters noticed anything that would presage the horrors police found on Newland Road.

They did not know the things his ex-wife knew and lived through, according to court documents: that Jones had problems with drugs and alcohol, that he struggled with mental illness, that he had attempted suicide, that he had abused her and made her afraid for herself and her two children. Then again, even Jones’s ex-wife appears to have underestimated his dangerousness in the end: He had regained the right to weekend visits with his older children.

We don’t know if Mei Jones was abused, or aware of her husband’s mistreatment of his first wife, before she died. We don’t brand abusers: They are free to move from one relationship to the next, hiding — or rationalizing — prior behavior in ways that convince other women to take chances on them. Still, if specialists had had a chance to look closely, they would have seen red flags in his story: a man previously accused of domestic abuse, with drug and alcohol issues, a history of depression and suicide attempts, and a wife who wanted to end the marriage.

In the aftermath of Monday’s discovery, the Arlington police chief took pains to reassure people that there was no public danger. “We’re quite certain the community is safe and people can go about their business,” he said. That was obviously true in this case, with the perpetrator dead by his own hand. But it is certainly not true for those with the most to fear. Many abuse victims cannot go about their business on any day, let alone after a day like Monday. They live in constant peril on that list, or in unmarked shelters with waiting lists hundreds of names long.

“Whenever something like this happens, it’s a huge sinking feeling here,” says Risa Mednick, who heads Transition House, which has a shelter in Cambridge. “For our clients, it’s an extra trigger. People become more cautious.”

And then there are the uncounted many, like Mei Jones, whose jeopardy was as invisible as it was extreme.

Since Monday, two more women have been added to the high-risk list in Cambridge, Arlington, and Belmont.